This invention comprises a method and apparatus for nondestructively analyzing a test object. The invention is especially useful for detecting minute defects in manufactured parts. Defects in objects usually induce strain anomalies which can be identified from the fringe patterns produced by this invention. The invention may also be used for other purposes. For example, the invention comprises an ultra-sensitive, whole-field strain gauge, which permits strain distribution of a large area to be measured without the need for conventional gauges or transducers.
One method, known in the prior art, for analyzing a test object is "shearography". According to this method, two laterally-displaced images of the object are made to interfere to form a pattern of fringes. The pattern is random, and depends on the characteristics of the surface of the object. When the object is deformed, by temperature, pressure, or other means, the random interference pattern will change. The amount of the change depends on the soundness of the object. A comparison of the fringe patterns for the deformed and undeformed states gives information about the structural integrity of the object. The method is called shearography because the one image of the object is laterally-displaced, or sheared, relative to the other image.
An example of a method for practicing the technique of shearography appears in U.S. Pat. No. 4,139,302, the disclosure of which is incorporated by reference herein. In the latter patent, the shearing is accomplished by placing a wedge-shaped prism along a portion of a lens. The light beams which pass through the prism are displaced relative to the beams which do not pass through the prism. Thus, the lens and wedge system produces two laterally-displaced images of the object.
The main disadvantage of methods for shearography of the prior art is the high spatial frequency of the patterns produced. Spatial frequency means the number of fringe lines per unit length. When the spatial frequency is too high, it is necessary to record the interference pattern on high-resolution photographic film. It will be shown later that the spatial frequency of a fringe pattern is given by
f=(2 sin (.alpha./2))/.lambda.
where
.alpha.=the angle made by two interfering rays, and
.lambda.=the wavelength of the light.
Because .lambda. is normally very small compared with the value of .alpha., the spatial frequency can become quite large. For example, if .alpha.=20.degree., and if .lambda.=0.5 microns, then the spatial frequency is about 700 lines per millimeter. It is not possible to view patterns having such fine detail with a video camera; instead, one must use a high-resolution photographic film.
In some cases, one might try to reduce the spatial frequency by reducing .alpha., such as by increasing the distance between the lens and the image. But the latter procedure would have the effect of greatly magnifying the image, and is therefore, at best, unwieldy, and, at worst, virtually unworkable. When the image and lens are moved far apart from each other, the image becomes so large that a video camera would need to scan the image almost point by point. Moreover, the overall intensity of the pattern would decrease, requiring a coherent light source of higher power. Also, in a wedge-shearing system, the interfering beams come from two halves of the lens, and thus the angle between the interfering beams is inherently large. Thus, the wedge-shearing technique inherently produces an interference pattern of high spatial frequency which is beyond the resolving capability of a video camera.
Another disadvantage of wedge-shearing technique is the need for optical filtering of the interference pattern. The high spatial frequency of the fringes produced by a wedge system makes the fringes very difficult to view with the naked eye. It therefore becomes necessary to provide an optical high-pass filter, which blocks out the low-frequency fringes, and which produces a pattern having visible dark bands corresponding to defective areas on the test object. Because it requires the use of photographic film, and because it also requires post-recording optical filtering, the wedge-shearing method is very cumbersome and slow when used for the inspection of objects. In general, it cannot fulfill the speed demands of a typical industrial production line.
Details of the high-pass optical filter used in the wedge-shearing technique are given in the article of Y. Y. Hung, entitled "Shearography: a New Optical Method for Strain Measurement and Nondestructive Testing", in Optical Engineering, May-June, 1982, vol. 21, No. 3, pages 391-395. The latter article is incorporated by reference herein.
One method of the prior art which avoids the problems due to excessively large spatial frequencies is the technique known as electronic speckle pattern interferometry (ESPI). An example of the latter technique is described in U.S. Pat. No. 3,816,649, the disclosure of which is also incorporated by reference herein. In ESPI, a beam of coherent light is directed onto the test object and reflected onto an image sensor. At the same time, a reference beam is also directed towards the sensor. The reference beam may be a "pure" beam or it may be reflected from a "reference" object. Both the object beam and the reference beam are nearly parallel, when they reach the image sensor, so the spatial frequency of the interference fringes is relatively low. Thus, the image sensor can be a video camera, or its equivalent.
While ESPI makes it possible to view an interference pattern directly with a video camera, it has important disadvantages. ESPI is similar to conventional holography, in that it requires an object beam and a reference beam of coherent light. The presence of two distinct beams increases the complexity of the optical system. The ratio of intensities of the object and reference beams must be carefully controlled, and the path lengths of the beams must be matched. Perhaps most importantly, ESPI, like holography, is very sensitive to vibration. The slightest movement of either the object or the apparatus for guiding the reference beam can ruin the pattern. Thus, ESPI requires special vibration isolation precautions, and is not practical for inspection of manufactured parts in a factory environment, or in the field.
Furthermore, ESPI measures absolute surface displacement, whereas the present invention measures relative displacement which is directly related to strains. Since defects in objects normally produce strain concentrations, it is easier to correlate defects with strain anomalies than with displacement anomalies.
The present invention overcomes the disadvantages of the prior art, described above, by providing a method and apparatus for analyzing a test object, without the need for photographic film, and using only one beam of light. With the present invention, the interference patterns can be recorded directly by a video camera, or other electronic image sensor, and processed by a computer, without intermediate developing and optical filtering steps. Thus, the invention can analyze objects at a video rate, i.e. up to about 30 frames per second. The output of the camera can be connected to a computer, which can store and analyze the data very rapidly. Thus, the invention is capable of inspecting objects at the rapid rate demanded by a typical production process.
Because only one beam is needed, the patterns obtained with the present invention are relatively insensitive to vibrations of the apparatus. The invention can therefore be used in typical production and field environments without special vibration isolation equipment. The single-beam process also eliminates the complex optical alignment problems associated with ESPI.
The present invention also provides means for measuring strains in a test object with extremely high precision. The invention can therefore be used as a whole field strain gauge, and is not limited to use in testing for defective objects.
The present invention also provides means for measuring the amplitude gradient in a steadily vibrating object. Measurement of gradients in the amplitude of vibration provides information on the maximum displacements of the vibrating object. The invention also provides means for measuring transient strains in an object.